Preheat oven to 200°c/180°c fan-forced. Line a large baking tray with baking paper.
Roughly chop half the sage leaves. Squeeze sausage mince from casings in a large bowl. Add beef mince, onion, chopped sage and 1 egg, mixing until combined.
Lightly beat the remaining egg in a small bowl, then set aside. On a flat surface, join pastry sheets along 1 side, overlapping by 2cm to form a large rectangle. Press firmly to secure. Spoon mince mixture along 1 long side of the pastry. Scatter over parmesan, then roll up the pastry from the long side to enclose filling.
Place sausage roll, seam-side down, on tray and brush with egg. Score pastry at 2cm intervals sprinkles over sesame seeds and season. Bake for 1 hour, adding remaining sage leaves in the last 10 minutes of cooking, or until filling is cooked and pastry is golden and puffed. Stand for 10 minutes to cool. Slice and serve with sage leaves and chutney.